The invention relates generally to an electrophotographic printing machine and, more particularly, to a development system which includes a magnetic auger assembly, mounted in a housing, for mixing and transporting developer materials.
Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced or is imagewise exposed by a raster scanned beam controlled by a digital image. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is formed on the photoconductive member, the image is developed by bringing a developer material into contact therewith.
Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attached to the latent image from the carrier granules to form a powder image on the photoconductive member which is subsequently transferred to a copy sheet. Finally, the copy sheet is heated to permanently affix the powder image thereto in an image configuration.
As the toner particles are depleted from the developer material, it is necessary to dispense additional toner particles into the developer mixture. Then newly added toner is typically mixed in some manner with the denuded carrier particles and unused developer material.
Various prior art devices have been devised to accomplish the mixing function. A preferred system for accomplishing the crossmixing function is the use of a dual auger system to transport the toner in two directions and achieve a toner interchange between augers. Dual auger systems are disclosed, for example, in the following prior art documents.
U.S. Pat. No. 4,274,362 to Beck et al. discloses magnetic brush mixing augers made of twisted strips of aluminum sheet metal with smooth axial edges. In a developing unit, the auger members are located in the sump portion of a developing pan where they circulate, distribute and intermix dry toner. A dispensing system evenly distributes regular amounts of toner while the copier is operable.
U.S. Pat. No. 4,056,076 to Smith, assigned to Xerox Corporation, discloses a crossmixing system for mixing and charging multicomponent developer in a circulating development system of an electrostatographic processor. A pair of parallel passive crossmixers are used as mixing devices and a single active crossmixer is used as a blending (triboelectric charging) device.
These prior art patents described above are representative of the dual auger crossmixing type of system. The common characteristic of these systems is the axis of each auger pair lie essentially in the same horizontal plane with developer exchange between each auger taking place at end locations. A problem with this inter-auger transfer is that the developer is exchanged by a sideways pushing application which requires that the augers be physically close to each other. For some systems, this proximity requirement may present a space or geometry problem. A second difficulty with this xe2x80x9cpushxe2x80x9d inter-auger transfer is the tendency for the developer to xe2x80x9cbunch upxe2x80x9d at the transfer end, sometimes resulting in toner spilling over into other areas of the developer housing unless specific seals are placed at strategic locations. A third difficulty is that systems having physical augers forces the material in the area of the sump where the auger is closest to the housing, causing packing of the material and in some instances failure of the auger or material property changes as the material binds together under pressure. Yet another difficulty is that the friction caused by systems which physically push powdered materials by contact augers causes heating and spatial non-uniformities, such as tunnels and tenting, in the materials being transported. This often can result in poor mixing, poor loading, and even toner block (fusing of the toner material in the housing instead of when heated on the print).
Applicant has also found that the physical separation of the toner into sections by fins in a physical auger can produce differential loading of the mag brush in an auger fed developer system and that the magnetic fields generated by the mag brush donor roll strongly affects the toner in the augers and appears to influence loading of the donor roll in a way to enhance the auger screw pitch separation. This has contributed to diagonal bands (auger mark print defects) being developed onto prints.
Briefly, the present invention obviates the problems noted above by utilizing a development system including a housing, a developer roll mounted in said housing, for depositing marking particles on an imaging surface having an electrostatic latent image thereon, an auger, for mixing and transporting the marking particles and carrier constituents of the developing material and; transferring mixed developer material to said developer roll, said auger comprising an elongate strip of magnetic material having a helix structure; a nonmagnetic sleeve enclosing said elongate strip of magnetic material; and means for rotating said elongate strip of magnetic material within said nonmagnetic sleeve, said elongate strip of magnetic material generates a magnetic field for transporting said developer material axially along an outer surface of said nonmagnetic sleeve when rotated by said rotating means. The system is uniquely characterized in that the preferred embodiment can be implemented with no moving parts of the auger touching the developer material while it is mixing and moving it.